ci: Use stable versions for reporting coverage

[matthewfeickert]

* Use a stable tag of codecov/codecov-action for reporting coverage.
* Report coverage for all Python versions tested.

[codecov[bot]]

Codecov Report

Base: 91.66% // Head: 91.66% // No change to project coverage :thumbsup:

Coverage data is based on head (1ac9c4d) compared to base (90e5ffe). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #46 +/- ## ======================================= Coverage 91.66% 91.66% ======================================= Files 3 3 Lines 12 12 ======================================= Hits 11 11 Misses 1 1 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `?` | | | unittests-3.7 | `90.90% <ø> (?)` | | | unittests-3.8 | `91.66% <ø> (?)` | | | unittests-3.9 | `91.66% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyhf#carryforward-flags-in-the-pull-request-comment) to find out more. Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyhf). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyhf)

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